The reductive mobilisation of iron from ferritin, the principal protein of iron storage, was studied. The kinetic characteristics of iron release by dithionite, thioglycollate, and dihydroriboflavin 5'-phosphate (FMNH,) were found to differ widely. The dependence on pH is most pronounced for the dithionite reduction which proceeds 100 times faster at pH 4 than at pH 7. The experimental data can be consistently explained in terms of specific interactions of products or educts with interfacial iron(II1) hydroxide of the ferritin core. Surface complexes with the product sulfite are postulated in the dithionite reaction, and with the educt in the thioglycollate reaction. Iron(I1) complexes with the radical anion FMN' are suggested to be involved in the iron release by FMNH?. The mobilisation of iron by a series of thiols of different size and coordinative properties confirmed the importance of surface complex formation. N o evidence was found for predominant effects of hindered shell penetration.Ferritin, as an iron storage protein, is a key molecule in iron metabolism. It maintains iron in a form from which it can easily be mobilised as and where it is required. Although the amino acid sequences of many ferritins are known [ I -51 and the three-dimensional structure of horse spleen ferritin has bccn solved at 0.28-nm resolution [6], the mechanisms of ferritin iron deposition and mobilisation are not yet really understood.

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